1. Field of the Invention
The present invention relates an electrolyte solution for a lithium secondary battery capable of forming a lithium secondary battery superior particularly in cycle characteristics and also superior in battery characteristics such as electrical capacity, storage stability.
2. Description of the Related Art
As the electrolyte solution for a lithium secondary battery, a nonaqueous electrolyte solution comprised of a solvent such as a cyclic carbonate, chain carbonate, ether, and a fluorine-containing electrolyte such as LiPF.sub.6 dissolved therein since it is suitable for obtaining a high voltage and high capacity battery.
However, a lithium secondary battery using such a nonaqueous electrolyte solution is not always satisfactory in terms of the battery characteristics such as cycle characteristics, electrical capacity, storage stability. An electrolyte solution for a lithium secondary battery which is superior in cycle characteristics and which does not cause the battery performance to decrease is desired.
Various methods have been proposed to improve the battery characteristics. For example, Japanese Unexamined Patent Publication (Kokai) No. 7-211349 discloses a method which treats a nonaqueous electrolyte solution, in which a fluorine-containing electrolyte is dissolved, with a fluorine adsorbent, that is, metal oxides such as MgO to make the content of the free acid (e.g. HF) 20 to 25 ppm. In the past, however, it has been known that a reaction of a metal oxide and HF causes the production of a metal fluoride and water. Further, it is known that water reacts with fluorine-containing electrolytes such as LiPF.sub.6, LiBF.sub.4, LiAsF.sub.6 to produce HF. Therefore, in the method of this publication, there is the concern that the reaction will cause the production of water, the hydrolysis reaction of the produced water and the LiPF.sub.6 in the electrolyte will again produce HF, and the amount of HF in the nonaqueous electrolyte solution will once again increase along with the elapse of time. Thus, there is a need for a further superior lithium secondary battery.
On the other hand, as a method for removing the diols from cyclic carbonate, there is known the adsorption method using an adsorbent such as silica gel, active carbon, activated alumina, or a molecular sieve. For example, Japanese Unexamined Patent Publication (Kokai) No. 5-74485 discloses a solution containing a cyclic carbonate as a solvent for a nonaqueous electrolyte. For making the concentration of the diols in the nonaqueous electrolyte solution a low one of not more than 1500 ppm, it describes a method of distillation of the cyclic carbonate and a method of treatment by an adsorbent. Further, a method of bringing a cyclic carbonate containing diols as impurities into contact with synthetic zeolite in the presence of a chain carbonate to remove the diols in the cyclic carbonate has been reported. That is, Japanese Unexamined Patent Publication (Kokai) No. 8-325208 describes a method of reacting ethylene glycol and a chain carbonate by zeolite in the copresence of a cyclic carbonate and chain carbonate to cause the production of a monoalcohol and removing that alcohol by a molecular sieve.